O,S-dialkyl phosphoramidothioates are effective insecticides. One particularly effective insecticide is O,S-dimethyl phosphoramidothioate (See, e.g., U.S. Pat. Nos. 3,309,266; 3,639,547; 3,309,266; and 4,389,350; the disclosures of which are incorporated herein by reference). U.S. Pat. No. 3,309,266 discloses that O,S-dimethyl phosphoramidothioate can be made by reacting O,O-dimethyl chlorophosphoro-thioate with ammonia or a primary alkylamine and then heating the product of that reaction in the presence of an alkylating reagent such as an alkyl halide. U.S. Pat. No. 3,639,547 discloses that O,S-dimethyl phosphoramidothioate can be made by reacting O,O-dimethyl phosphoramidothioate with the dimethyl ester of sulfuric acid or with a methyl ester of organic sulfonic acids. The reaction occurs at a temperature of from about 20.degree. C. to about 100.degree. C. In a manner similar to the method disclosed in U.S. Pat. No. 3,309,266, the O,O-dimethyl phosphoramidothioate can be made via ammoniation of a O,O-dimethyl halophosphorothioate. U.S. Pat. No. 4,389,350 discloses a catalytic isomerization of O,O-dimethyl phosphoramidothioate to O,S-dimethyl phosphoramidothioate wherein the reaction mixture is separated into "starting material" and isomerized product, to effect further isomerization of the "starting material".
With the above methods, the yield and purity of the formed O,S-dimethyl phosphoramidothioate are low, ranging from about 30% to about 70%, or the purification process is extensive requiring repeated isomerization steps. There continues to be a need in the art, therefore for an efficient method for making O,S-dimethyl phosphoramidothioate at an increased yield and purity.